It is known in principle that low frequency base signals of several origins can be converted into higher frequency signals by means of carrier frequencies. Such higher frequency signals can be transmitted via a common transmission path (either wire or wireless) in the form of a wide band transmission line or as a frequency band of a radio transmission.
With signals so converted, a mix of the higher frequency signals occurs in the transmission path. At the end of the transmission path, each individual higher frequency signal must be filtered out before it can again be converted (by means of the carrier frequency allocated to it) into the original low-frequency base signal. Such processes have to take place without substantially impairing the "intelligibility" of the signal and without significant "cross talk" between the individual channels.
During conversion, two sidebands of the same channel are created (a lower and an upper sideband), one each on either side of the carrier frequency. Either or both sidebands can be transmitted. And the carrier itself can be transmitted or such carrier can be suppressed on the transmission side. When suppressed, a new carrier of the same frequency has to be re-created at the receiving side, in order to re-convert the respective signal into the low-frequency base signal.
The aforementioned single-sideband transmission has been proven to be successful in utilizing the full "width" of the available transmission band of the common transmission path and yet spacing the channels as closely as possible, thereby obtaining as many channels as possible in the available band. However, it requires higher expenditure for high quality filtering circuits at the end of the transmission path, in order to filter out the only sideband and to regain the original base signal without loss of information. In addition and in the case of single-sideband reception, the demodulation of the signal cannot be performed through simple rectification, as in the case of double-sideband oscillations. Rather, demodulation is by multiplying the high-frequency signal and the intermediate-frequency signal by an oscillation, the frequency of which must be exactly equal to the frequency of the carrier.
The technology involved in double-sideband receivers, which is comparatively simple, is believed to be the reason why single-sideband technology has not found favor for use with the internationally-recognized wave plans, especially in the area of short, medium and long wave radio transmission. Moreover, the double-sideband transmission often has an advantage for the owners of single-sideband receivers. If one sideband is subject to interference from an adjacent signal, the receiver user can switch reception to the other sideband, providing the latter is substantially interference-free. This is sometimes referred to as diversity reception. Therefore, double-sideband transmission arrangements continue to exist.
However, compared to the conditions in the case of single-sideband transmission, there is a requirement to reduce the number of channels in the frequency band of the transmission path in half or to narrow the individual channels with regard to their low-frequency band width. If, however, the same number of channels (such number often being the same as with single-sideband transmission at nearly equal band width) is transmitted, there will almost certainly be overlapping of those sidebands of two double-sideband oscillations which are adjacent to one another. That is, the upper sideband of one oscillating channel will overlap and interfere with the lower sideband of the next-higher oscillating channel of different origin.
With such two channels of different origin, it is not possible (or at least not easy) when receiving the signals to filter out at least one of the sidebands from the "mix" of overlapping sidebands. As a consequence, when the allocated carrier is added according to the principle of single-sideband reception, such reception is not substantially free of interference. Re-conversion into the desired basis signal is, at the least, very difficult.
With regard to the state of technology concerning the cancellation of interfering signals, reference is made to the following publications. German patent document DE 22 33 614 A1 depicts a circuit arrangement for the reduction of the interference output in code multiplex transmission devices. In a receiver for code-multiplex signals, interfering, non-orthogonal signals by other transmission stations are selected and subtracted from the composite signal mixture by using the "not code pattern of the desired transmitter" feature.
German patent document DE 28 52 127 A1 depicts a device for the suppression of an undesirable signal. In a receiver, vestigial-sideband modulated signals (e.g., in a TV set) and interfering signals, offset in time but of the same type (such as echo signals), are expediently selected and subtracted from the composite signal mixture by using the feature "not first signal in time."
German patent document DE 26 22 058 A1 depicts a procedure for eliminating intelligible cross-talk during the transmission of information through transformation of amplitude and frequency modulation by means of frequency modulation frequency division multiplex or FM-FDM. In a receiver for frequency modulated signals (in the context of an FM-FDM transmission system), the interference signals "intelligible cross-talk" which appear "downstream" of the frequency demodulator are created a second time at the receiving end. This is by a special amplitude demodulator which is connected in parallel with the actual frequency demodulator. The interference signals are then subtracted from the output signal of the frequency demodulator.
The technology of the aforementioned German patent documents shares some commonality. Interfering signals are eliminated through subtraction of the selected or reproduced interfering signal. However, such technology does not involve overlapping sidebands of adjacent transmitters. The procedures described in such documents is not suitable for canceling interfering, overlapping sidebands.
Another document which deals with the cancellation of interfering signals is an article by P. L. Taylor titled "Eliminating Adjacent-Channel Interference" published in Wireless World, vol. 83, No. 1499, July 1977, pages 55 to 57. The article describes the following situation. There are a desired signal in the form of a double-sideband oscillation and an interference signal in the form of a double-sideband oscillation. The carrier oscillations of the two double-sideband oscillations are so close in their frequencies that the sidebands not only overlap each other, they extend beyond the carrier of the respective other double-sideband oscillation. Both outer companion sidebands must be free of interference; as such, they must not be interfered with by an overlapping signal on the upper or on the lower side. The article mentions two procedures to eliminate the interfering signal.
In the first procedure, the sum of the desired signal and the interfering signal is product-demodulated with an oscillation with respect to which the frequency and the phase of the carrier oscillation of the desired signal must coincide exactly. The result is that the demodulated base band of the desired transmitter is heterodyned by an interfering double-sideband oscillation, the frequency-converted carrier oscillation of which together with the two sidebands come to be located, in an interfering manner, in the demodulated base band of the desired transmitter. The frequency-converted carrier frequency of the interfering signal is equal to the difference of the "carrier frequency of the interfering signal minus carrier frequency" of the desired signal.
That part of the lower sideband of the lower frequency interfering oscillation, which becomes mathematically negative during demodulation, appears--reflected at frequency zero--from zero on upwards. For reasons of clarity it should be stated that the demodulated base band of the desired transmitter--as is the case with every synchronous demodulation--represents the in-phase sum of the two equal sidebands of the desired signal that add up to be positive. This base band is now interferingly heterodyned by the described double-sideband oscillation with the reflected part. Freeing the demodulated base band from the latter is the purpose upon which the first procedure described by Taylor is based.
The task is solved in such a way that, in a second signal path, the original sum of the desired signal and the interference signal is, firstly, product-demodulated with the carrier of the desired transmitter, phase-shifted by 90.degree., whereby--as is known from quadrature modulation systems--the desired signal is canceled and the interfering signal remains. Subsequently, the interfering signal is multiplied by a special square-wave oscillation in a manner such that the mentioned double-sideband oscillation with the reflected part is achieved. It is then subtracted from the disturbed demodulated base band of the first signal path.
To put it another way, the characteristic of the procedure according to Taylor consists of the design and creation of the special rectangular function, whereby the reflected part is obtained with the correct phase and sign. In the case of plain multiplication with a sine wave or a normal rectangular function, this would not be true.
The procedure always works with complete double-sideband oscillations. From this it follows that, as already expressed above, in the first procedure described by Taylor, the interfering signal which is to be canceled must not only be a complete double-sideband oscillation, but must itself not be interfered with by another interfering signal on one of its sidebands.
According to the second procedure described by Taylor, the sum of the desired signal and interfering signal is product-demodulated with the carrier oscillation of the interfering signal which is phase-shifted by 90.degree.. Thereby, the interfering signal is canceled. Because the demodulation products of the upper and the lower sideband are then two base bands with opposite signs, they cancel each other out. (This technique is also known in quadrature modulation systems.) That which remains is the frequency-converted desired signal. However, such desired signal remains with a carrier frequency which is equal to the difference of the carrier frequency of the interfering signal minus the carrier frequency of the desired signal.
This double-sideband oscillation also contains a reflected part. Therefore, a normal demodulation is not possible. In this case as well, this is accomplished by a special rectangular oscillation with which multiplication is performed for the purpose of demodulation. In principle, it is the same as in the first procedure but involves a symmetrical function in relation to the ordinate axis, whereas the rectangular function of the first procedure was obliquely symmetrical in relation to the zero point. Also for the second procedure described by Taylor, the interfering signal must not be interfered with by another signal on any of its sidebands. The procedures described by Taylor take into consideration only the case of a single interfering signal.